This invention relates to rotary magnetic bearings of the null flux type in which there is relative rotation between (a) a member which carries a plurality of loops of electrically conductive material and (b) a plurality of field magnets which produce magnetic fields. Each loop is simultaneously exposed to two oppositely directed magnetic fields. When the loop is on its prescribed path, it is in a null flux (zero net magnetic flux) condition because it is exposed to equal quantities of the oppositely directed magnetic fluxes. However, when the loop deviates from the prescribed path, it is exposed to unequal quantities of the oppositely directed fluxes. There will be a finite net magnetic flux, thus resulting in the production of electromotive forces and an induced electric current in the loops. The direction of this current, in the presence of the magnetic fields, exerts Lorentz forces on the loops in directions which are lateral with respect to the circular path.
A detailed description of the principles of this type of bearing are provided in U.S. Pat. No. 5,305,874 which is incorporated herein by reference.
In bearings of this type, the loops of a radial bearing should all travel in a prescribed circular path which is concentric with a prescribed axis of rotation. In an axial bearing, the axial positioning of the loops should be uniform.
It is desirable to avoid any current flow in the loops when they are traveling in the prescribed path, but some obstacles have been experienced in achieving this condition. In manufacturing procedures, it is difficult to achieve perfect positioning, so loops are often offset from their designed positions. This results in a lack of uniformity so that all loops in a radial bearing are not equidistant from the axis of rotation, and all loops in an axial bearing are not at a uniform axial position. Due to this lack of uniformity, some current will flow in some loops at all times, even when the rotor is at its prescribed position.
In a radial bearing, centrifugal forces can also result in undesired loop currents. In such bearings, the loops are carried by a disc. When the disc rotates at very high angular velocities, centrifugal force will cause radial expansion of the disc. For example, a 4 inch diameter, 0.1 inch thick disc of fiberglass rotating at 60,000 RPM will expand radially almost 0.02 inch. Thus, even though the rotor axis is at its prescribed position, some current will flow through the loop. Under this condition, the loop will generate heat, the bearing will lose some of its stiffness, and the shaft will be subjected to a steady drag to detract from bearing performance.
Another source of undesired loop current and potential overheating in a radial bearing is attributable to the geometry of the loops and the magnetic fields. The radial legs of a loop do not evenly enter and exit the magnetic fields. The boundaries of a magnetic field have their radial borders at one angle, while the radial legs of a loop can have a range of angular borders which vary from the leading edge to the trailing edge of each radial leg. Due to this geometrical situation, a voltage may be prematurely induced in a radial loop, and the resulting current can continue for a significant period. Unwanted heating due to these currents is thus generated.
The present invention provides a solution to the foregoing problems, and it will minimize the occurrence of undesired currents in the loops of a rotor which is rotating at its prescribed axial and radial position.